Recently, membrane-based gas separation is drawing attentions as a rapidly growing separation technology. Gas separation using membranes has many advantages over the existing separation processes, including high-level process availability, low energy consumption and low operation cost. Particularly, there have been many basic researches on organic polymer membranes since 1980s. However, the traditional polymer exhibits relatively low material transport rate because it has few micropores.
In contrast, polymers having high level of free volume, known as microporous organic polymers, which can adsorb small gas molecules and exhibit improved diffusion capacity, are emerging as leading candidates in separation processes. Therefore, based on the fact that microporous polymers having a rigid ladder-like structure that prevents effective packing of the polymer chain exhibit relatively high gas permeability and selectivity, various researches are being conducted to develop organic polymers that can be used for gas separation membranes.
For example, efforts are being actively made to use rigid, glassy, pre-aromatic organic polymers exhibiting superior thermal, mechanical and chemical properties such as polybenzoxazole, polybenzimidazole, polybenzothiazole, etc. as gas separation membranes. However, because these organic polymers are mostly hardly soluble in general organic solvents, it is difficult to prepare membranes by the simple and practical solvent casting method. Recently, the inventors of the present disclosure reported that a polybenzoxazole membrane prepared by thermally rearranging polyimide having hydroxyl groups at ortho positions exhibits 10-100 times higher carbon dioxide permeability as compared to the existing polybenzoxazole membrane prepared by the solvent casting method. However, the carbon dioxide/methane (CO2/CH4) selectivity is still comparable to that of the existing commercially available cellulose acetate and needs to be improved (non-patent document 1).
The inventors of the present disclosure also reported that introduction of benzoxazole groups to the hydroxypolyimide copolymer membrane through thermal rearrangement leads to increased rigidity of the polymer chain, thereby improving gas separation performance owing to increased free volume. However, if the content of the benzoxazole groups introduced into the polymer chain is 80% or higher, the resulting membrane may break easily and exhibit poor mechanical properties because it is too hard. Also, a large-area membrane may exhibit unsatisfactory gas permeability and selectivity owing to shrinkage caused by release of a large quantity of CO2 during the thermal rearrangement (non-patent document 2).
Also, it was reported that a polybenzoxazole membrane prepared by thermally rearranging a blend membrane of polyimide having hydroxyl groups at ortho positions and poly(styrene sulfonate) at 300-650° C. exhibits up to about 95% improved carbon dioxide/methane (CO2/CH4) selectivity as compared to the polybenzoxazole membrane prepared by thermally rearranging hydroxypolyimide not containing poly(styrene sulfonate). However, since a method of synthesizing the polyimide as a precursor for preparing the polybenzoxazole membrane is not specified, the fact that the free volume element and gas separation performance of the thermally rearranged polybenzoxazole membrane can vary depending on the imidization method of the hydroxypolyimide, i.e., solution thermal imidization, azeotropic thermal imidization, solid state thermal imidization or chemical imidization, is not considered at all (patent document 1).
In consideration of the fact that the properties of thermally rearranged polybenzoxazole are affected by the synthesis method of the aromatic polyimide, polyimides having hydroxyl groups at ortho positions were synthesized using various methods including solution thermal imidization, solid state thermal imidization, chemical imidization, etc. and polybenzoxazole membranes were prepared by thermally rearranging them. However, the resulting membrane is for use as a separation membrane for dehydration of ethanol or other organic solvents utilizing its superior separation characteristics derived from its peculiar porous structure, not as a gas separation membrane (patent document 2).
Also, it was reported that a polybenzoxazole membrane prepared by synthesizing polyimide having hydroxyl groups at ortho positions by chemical imidization followed by thermal rearrangement and UV irradiation to form a crosslinked structure exhibits improved selectivity. However, because the polyimide is prepared by chemical imidization, not by thermal imidization, the resulting thermally rearranged polybenzoxazole membrane still has relatively low carbon dioxide permeability. In addition, the process is disadvantageous in that a UV radiating apparatus has to be used to form the crosslinked structure (patent document 3).
Noting that the mechanical properties, membrane area shrinkage and gas transportation behavior of a thermally rearranged polybenzoxazole membrane are determined by the imidization method of polyimide, the benzoxazole group content in the polymer chain and the crosslinked structure of the polymer chain, the inventors of the present disclosure have found out that a crosslinked, thermally rearranged polybenzoxazole membrane can be obtained by synthesizing a polyimide membrane having hydroxyl and carboxylic acid groups in the polyimide repeat unit through solution thermal imidization and then simply heat-treating the same.
Also, they have found out that a polybenzoxazole membrane having a crosslinked structure with a benzoxazole group content of less than 80% in the polymer chain, which is prepared by synthesizing a copolymer having hydroxyl and carboxylic acid groups in the polyimide repeat unit and having a hydroxypolyimide content of less than 80% in the polyimide repeat unit by solution thermal imidization followed by chemical crosslinking and then thermal rearrangement, or chemical crosslinking and thermal rearrangement at the same time, exhibits remarkably improved separation performance as a gas separation membrane due to superior mechanical and thermal properties.